Many of these people not have 240Hz monitor, so i doubt they can provide the answer u looking for. I for one have been using it for 1 month now (Alienware AW2721D). I'm using 3 monitors at once (another 2 of them is 144hz & 75Hz). Cpu is ryzen 9 5900x & gpu rtx 3090 Ti evga ftw3 ultra (rip evga GPU businesses, they have issues with Nvidia).
Even now, 240Hz give my brain a hard time to focus. 240Fps is too smooth & fast. No matter games i played (warframe, destiny 2, sword art online alicization, all 8 yakuza, & another 100+ games), give me same experience of hard time focusing.
But once i lower down the Hz to 120Hz or play heavy games like cyberpunk, metro exodus enhanced edition, red dead redemption (games that lower fps to 140 & below), I'm having no issues playing normally.
Thus, in my case, it's the refresh rate giving me issues. So, i know this is a lot to ask with how expensive 240Hz monitor u bought, but reducing the Hz of monitor might be the only thing that can cure your issue or maybe train ur brain to at least 6 months to become familiar with it.